Just like 99% of the China phone companies, they are short sighted and only make GSM compatibility. Verizon and Sprint are CDMA. With the new Verizon plans, there is an incentive to BYO phone to the Prepaid side. This is because nice phones now cost $600 on the pay monthly plan. No more $100-200 deals.
So, why would these manufacturers ignore 50% of the US market?
While my first impression is this is a wasted effort by ZTE on further reflection, it's not. This could be a part of a larger push to get flagship phones into the hands of consumers for less money. That said, it doesn't seem like that good of an effort.